Solve for $t$ : $21 = t - 4$
Add $4$ to both sides: $ \begin{eqnarray} \\ \hphantom{21 {+ 4}} &\hphantom{=}& \hphantom{t} \\ 21 &=& t - 4 \\ \\ {+4} && {+4} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 21 {+ 4} &=& t \\ \end{eqnarray} $ Simplify. $ t = 25$